The invention relates to electromagnetic actuators for moving an armature carrying a drive rod in linear translation along the axis of the rod. A particularly important but non-exclusive application of the invention lies in actuators for bringing a valve alternately into an open position and into a closed position, and in particular to actuators for the valves of an internal combustion engine using spark ignition or compression ignition.
French patent application No. 98/12489 (FR-A-2 784 222) describes an electromagnetic actuator having, in a housing, a ferromagnetic circuit defining an axial displacement interval for a rod-driving ferromagnetic armature between two extreme positions in which the armature bears against poles of the ferromagnetic circuit, resilient return means being provided for holding the valve at rest in a middle position between the extreme positions, and at least one coil carried by the circuit and enabling the armature to be brought in alternation into the two extreme positions.
The electromagnetic means can comprise two coils placed on either side of the armature for which excitation attracts the armature respectively in a direction tending to close a valve, and a second electromagnet placed on the other side of the armature which, when excited, tends to bring a valve into a fully open position, for example. The embodiment described in patent application No. 98/12489, to which reference can be made, has, on the contrary, only a single coil mounted on the ferromagnetic circuit which is of a structure such that in combination with the armature it presents two stable magnetic flux paths both corresponding to an air gap of zero size between the armature and one of the poles of the ferromagnetic circuit.
Satisfactory operation of such an actuator requires initial adjustment so that the armature is at rest in a middle position between its extreme positions. For this purpose, adjustment members can be provided for adjusting the initial compression of one of the springs, e.g. means such as those described in the French patent application filed on the same day as the present application for xe2x80x9cDispositif rxc3xa9glable de commande de soupages et procxc3xa9dxc3xa9 de rxc3xa9glage d""un tel dispositifxe2x80x9d [An adjustable valve control device and a method of adjusting such a device]. However it is necessary to have a sensor for sensing the position of the armature to make it possible to determine the position of the armature in the interval or air gap defined by the poles. Furthermore, good operation requires the energy delivered to the coil(s) to be sufficient to guarantee that the stroke of the armature is complete, but not excessive so as to avoid end-of-stroke impacts which would generate noise and wear.
To solve the second problem, application 98/12940 ensures that the energy applied during the final stage of armature displacement is determined by measuring the reluctance of the coils, which implies a ferromagnetic circuit such that there exists an almost linear relationship between the reluctance R(x) and the air gap x during the last fractions of the stroke prior to the armature sticking against the poles of the ferromagnetic circuit. That approach does not make it possible to measure the rest position of the armature.
The invention seeks in particular to provide an actuator of the above-defined type provided with means making it possible to determine the rest position of the armature inaccurate manner.
To this end, the invention provides in particular an actuator whose rod or housing carries a radially magnetized bar of length not less than the travel of the armature, and in which the housing or rod carries at least one magnetic flux sensor placed in a zone which is weakly exposed to the fields induced by current passing through the coil. The sensor can be a Hall effect sensor, in particular.
A Hall effect sensor has a response that is substantially linear as a function of field, thus making it possible to track travel of the magnet by measuring its output signal. Furthermore, sensor drift whether due to temperature or aging is slow, which means that recalibration need be performed only periodically in order to identify the signal corresponding to the armature being in its middle position.
The bar can be fixed to the rod, which facilitates the requirements of the sensor. In order to reduce sensitivity to alternating accelerations, the disposition can be inverted.
In order to reduce the effects of any external disturbing components, while also increasing the useful signal, the detector can have two sensors whose sensitivity directions are opposite and which are placed on either side of the rod, with a subtracter receiving the outputs from the two sensors. Thus, external effects which are equivalent on both sensors cancel.
When both sensors are carried by the housing, they can be placed side by side on a common silicon substrate, with ferromagnetic circuits conveying fluxes sensed on either side of the rod to respective ones of the sensors and with a subtracter receiving the outputs from the two sensors.
The invention also provides a method of adjusting an actuator, comprising the steps of:
bringing the armature into one of its extreme positions by feeding the or one of the coils and measuring the output signal from the sensor;
taking the armature to its other extreme position and measuring the output signal from the sensor; and
determining the output signal corresponding to the middle position of the armature on the basis of the measured signals.
The bar can be on the rod and the sensor on the housing. The disposition can be inverted, in order to accommodate magnet fragility.
For an actuator having a single coil, of the kind described in application 98/12489 (FR-A-2 784 222), the intensity of the magnetic field in the plane of symmetry of the magnetic circuit containing the axis of the armature is small enough for it to be possible to place the sensor therein without taking special precautions. The normal to the plane of the sensitive element of the probe is placed in said plane of symmetry. In contrast, in a circuit having two coils, where the two coils are oriented in planes orthogonal to the displacement axis, it is generally necessary to shield the ends of the coils, e.g. by making the yoke of the actuator out of ferromagnetic material.
The above characteristics and others that are advantageously usable in combination with the preceding characteristics but which are capable of being used independently will appear better on reading the following description of a particular embodiment given by way of non-limiting example.